It is widely thought that the exposure of chondrogenic factors to bone marrow progenitor cells in cartilage lesions can enhance the synthesis of a repair tissue. This proposal is based on the hypotheses that gene transfer can be used as a means to achieve sustained synthesis of specific proteins within a cartilaginous lesion, and that this can be used to augment the differentiation of mesenchymal stem cells toward chondrogenesis in vivo. We have recently found that a natural clot created from freshly coagulated bone marrow aspirate forms a simple, but surprisingly effective, matrix for delivery of gene transfer vectors and genetically modified cells to osteochondral defects. The matrix formed is completely native to the donor and part of the natural reparative process. From our preliminary results we have found that in culture mesenchymal stem cells within coagulated bone marrow aspirates will undergo differentiation into cartilaginous tissue under the proper cytokine stimulation. Furthermore exogenous transgenes seeded into the clot as vectors or genetically modified MSCs will be expressed for several weeks. The experimental scheme of the proposed study is designed to broaden our knowledge of the role of protein factors in the process of chondrogenesis while laying the foundation for development of gene- and stem cell-based approaches to improve the natural repair of cartilage lesions. For this we will address the following Specific Aims: (1) to determine the appropriate vector for delivery of chondrogenic transgenes to MSCs; (2) to evaluate the ability of candidate transgenes to stimulate MSCs toward chondrogenesis within a pellet culture system; (3) to optimize the delivery of chondrogenic genes to bone marrow clots in vitro; (4) to characterize patterns of transgene expression in vivo following implantation of genetically modified bone marrow clots in osteochondral defects; (5) to evaluate the capacity of genetically modified bone marrow clots to induce chondrogenesis in osteochondral defects in vivo; and (6) to elucidate the biological effects of long-term expression of chondrogenic transgenes within the joint.